L or (S)-3-(3,4-dinydroxyphenyl)-2-methylalanine, more commonly known as methyldopa, is a well-recognized antihypertensive agent which is generally administered orally in the form of capsules, pellets, or emulsions. Certain esters of methyldopa have been found to be of greater potency, Saari et al., J. Med. Chem. 21, 746 (1978). One of the esters found to be of higher potency is the 1-pivaloyloxyethyl ester of (S)-3-(3,4-dihydroxyphenyl)-2-methylalanine. Preparation and pharmaceutical applications of these pivaloyloxyethyl esters are also described in U.S. Pat. Nos. 3,983,138; 3,988,341; 4,016,288 and 4,051,169. The latter patent also teaches that both isomers arising as a result of the chiral center in the substituted alkyl portion of the ester are active pharmacologically. Subsequent work has supported the efficaciousness of the pivaloyloxyethyl ester and the desirability of employing this highly effective ester product in special means for administration such as in controlled delivery systems. Controlled delivery systems include not only a delayed delivery method such as enteric coated tablets but also and particularly, delivery in devices such as an osmotic pump for controlled and continuous release of drugs. A representative device is described in U.S. Pat. No. 4,265,874. For efficient and effective use in such device, it is highly desirable that the drug be in a pure crystalline form, and further exhibit appropriate solubility properties for controlled release in an aqueous environment. However, the pivaloyloxyethyl ester tends to be unstable even when obtained in a crystalline form. Thus, it is desirable to provide a method for obtaining the pivaloyloxyethyl esters of methyldopa not only in a crystalline form, but also in a stable form. Further, it is desirable to have the ester in a form having solubility properties adaptable for controlled release in an aqueous environment. Moreover, since as taught in U.S. Pat. No. 4,051,169, both isomers of the esters are pharmacologically active, it is desirable to provide for a method in which both isomers may be obtained in crystalline form without undue difficulty.